Conventionally, in order to synchronize the energization of the different phases of a three-phase motor with the rotation of the rotor of the motor, it is considered necessary to provide the same number of sensors as there are phases to be synchronized, the purpose of these sensors being to identify the instant rotational position of the rotor.
An example of this is shown in FIG. 1 of the accompanying drawings, which illustrates the simplest case of a three-phase synchronous motor, the rotor of which has only one pair of poles. In this case, the electrical angle is equal to the mechanical angle, that is to say one period in the control signals corresponds to one revolution of the rotor.
The windings B1, B2 and B3 of the stator of the motor are for example energized by three energized signals G1, G2 and G3, such as are shown in FIGS. 2a to 2c of the accompanying drawings. Three sensors C1 to C3 (FIG. 1) are usually provided for generating the three signals G1 to G3. The sensors follow the rotation of the rotor and detect the instant at which a transition must occur on the energising signals G1 to G3 respectively.
The sensors C1 to C3 are for example Hall effect cells which are spaced apart around the stator of the motor, the sensors being offset from each other by 120 degrees as shown in FIG. 1. The rotor also includes a magnetic sector S with an opening (or angle subtended at the centre) of 120 degrees. This sector cooperates with the three sensors C1 to C3.
By convention, it is decided that the winding directions of the windings are such that when a current flows through a winding, it tends to cause the north pole of the rotor to be aligned on its axis.
The signal which is given by each sensor Ci is the signal Gi which must be applied to the corresponding winding Bi. On starting, it is sufficient to energize whichever one of the windings is associated with the sensor which is in the high state, whereupon the motor will start.